Don't screw into the roof!
There should be some metal brackets that attach to the aluminum vent portion. To be honest I'd get the brackets that have a hinge, as you will want/need to clean the cover and the vent cover. I used to have the type you posted, but I've gone to the one below MaxxAir2. As it lets more air flow if you ever install a power vent, with the one you have you're fine. But you need the metal flanges, you can see them pictured below. If you didn't get them go back and ask for them
Download the instructions page
here.